Expand / Add definition around route_type

## Customer Need

*On behalf of Amtrak...*

*As a customer, I'd like to know (and potentially select a route by) if I can expect the bus route I've chosen to provide a comfortable amount of space to sit and some space for my personal items - so that I can evaluate if it is an appropriate route for me or if is worth any difference in costs.*

*As a customer, I'd like to know (and potentially select a route by) if I can expect the bus route I've chosen to requires me to navigate across vehicle rows where I cannot stand and sit, next to individuals that I may not know without the option to move during the trip - so that I can evaluate if it is an appropriate route for me (my ability and comfort level in being in close proximity to others) or if is worth any difference in costs.*

## Current Spec

[`routes.route_type`](https://gtfs.org/reference/static#routestxt) :
> Indicates the type of transportation used on a route. Valid options are:
>
> 0 - Tram, Streetcar, Light rail. Any light rail or street level system within a metropolitan area.
> 1 - Subway, Metro. Any underground rail system within a metropolitan area.
> 2 - Rail. Used for intercity or long-distance travel.
> 3 - Bus. Used for short- and long-distance bus routes.
> 4 - Ferry. Used for short- and long-distance boat service.
> 5 - Cable tram. Used for street-level rail cars where the cable runs beneath the vehicle, e.g., cable car in San Francisco.
> 6 - Aerial lift, suspended cable car (e.g., gondola lift, aerial tramway). Cable transport where cabins, cars, gondolas or open chairs are suspended by means of one or more cables.
> 7 - Funicular. Any rail system designed for steep inclines.
> 11 - Trolleybus. Electric buses that draw power from overhead wires using poles.
> 12 - Monorail. Railway in which the track consists of a single rail or a beam

## Proposed Solutions

#### 1. Add `coach` as a `route_type` | Update `bus`

3: Bus,
- customers may be seated or standing
- differentiated from a van by customers being able to navigate across rows via an aisle that is standing-height

9: Coach,
- a bus with premium features such as:
- more comfortable, dedicated seating.
- May or may not include: space underneath and above the rider for luggage, individual climate control, on-board restrooms, entertainment, and connectivity.

#### 2. Add `van` as a `route_type`

8 : van, a slightly larger auto where
- a single or pair of doors provides access to multiple rows of seats, access to which is typically provided via a small side aisle or folding seats.
- Interior ceiling is below typical standing height.

#### 3. Add `BRT` as `route_type`

10: Bus Rapid Transit (BRT), a bus service (bus, coach, or electric trolleybus) with supporting infrastructure designed to improve travel time and service reliability where:
- all or a significant portion of the route operates on dedicated transit right-of-way;
- dwell times are minimized through strategies such as all-door-boarding, ticket pre-payment, and level-boarding; and
- travel times are enhanced using strategies such as transit signal priority or higher-speed roadway design.

BRT may or may not include enhancements to:
- customer communication and branding; and
- on- and off-board rider amenities.

We could also just reference: ["What is BRT"](https://www.itdp.org/library/standards-and-guides/the-bus-rapid-transit-standard/what-is-brt/), ITDP

## Previous discussion

#174 considered several additional `route_type` values and settled on adding one for monorail and one for trolleybus. There is a lot of great discussion in the issue thread that is worth reviewing.
- Intercity bus/coach was not added at the time due to concerns that it was:
- Potentially not backwards compatible because many feeds currently code intercity bus as bus
- *I see this is non-breaking because old versions of the feed would still "make sense" to users and would not fail validation*
- *Arguably trolleybus would have had the same issue?*
- Lack of specificity in the definition of intercity bus
- *I think we can get to a good definition.*

I am unaware of any previous discussions about "vans" (vs autos) but would welcome them.

## Relevant Pull Requests

#279 has the text for all previous proposed extended route types

## Relevant open proposals

The following proposals are also relevant:

- [GTFS-Vehicles](http://bit.ly/gtfs-vehicles)
- proposes very granular features available on various vehicle types.
- **This proposal does not inhibit any of the GTFS-Vehicles proposal**
- [GTFS-ModesAndNetworks](https://docs.google.com/document/d/1qWFZ8bYD0d4CVtNfXjzKyq3nBYEGIoDZREayseDnrok/edit#)
- Considers additional expansion of `route_type` and grouping of routes and route types into networks and modes.
- **This proposal does not inhibit any of the GTFS-ModesAndNetworks proposal** and provides a lower-effort shortcut to achieve several of its basic user needs.
- [GTFS-OnDemand](https://bit.ly/gtfs-ondemand)
- Proposes the addition of an enum value for car/auto/van.
- **This proposal would modify the GTFS-OnDemand definition of car/auto to exclude "van"**